1. Field of the Invention
This invention relates to an apparatus and a method for radiation measurement that are suitable for use within and outside nuclear reactor or accelerator facilities when there is a need to perform real-time monitoring of radiation intensity over a wide dynamic range at many and broadly distributed sites of measurement. In nuclear reactor and accelerator facilities, one must use a detection system with which radiation doses ranging from a very weak level to a very intense level can be monitored at all times in various places. A portable radiation monitor is also required for personal radiation exposure management.
To meet these needs, the present inventors developed a simple differential and integral-type apparatus and method for radiation measurement that were based on the combination of two principles, one performing constant radiation monitoring and the other integrating measured radiations with a stimulable phosphor. The present inventors also developed an apparatus and method for radiation measurement that was applicable to a remote sensing radiation measuring apparatus, a radiation distribution measuring apparatus and a portable radiation measuring apparatus.
2. Prior Arts
Conventionally, radiation measurement has been performed with an ionization chamber, a Geiger-Mueller counter (GM counter) and a scintillation detector consisting of a scintillator combined with a photomultiplier tube whereas neutron dosimetry has been conducted with a BF3 or 3He counter. However, measuring a wide dynamic range of radiation doses from a very weak level to a very intense level by a single detector is extremely difficult and to overcome this difficulty, an ionization chamber of low sensitivity has been used in combination with a scintillation detector of high sensitivity. High-intensity radiation doses that occur instantaneously around an accelerator or a target or which result from an unexpected accident in nuclear reactor facilities have also been difficult to measure since they saturate most detectors.
A stimulable phosphor has two actions, one is accumulating an incident radiation and outputting the quantity of accumulated radiation as stimulated fluorescence upon stimulation by exciting light and the other is emitting prompt fluorescence upon excitation by an incident radiation. On the basis of these two actions, a method was developed that could measure the quantity of incident radiation by selectively detecting stimulated fluorescence and prompt fluorescence at specified time intervals [Japanese Patent Application No. 50301/1999]. By using an apparatus for radiation measurement that relies upon this technique of time-division multiplexing, a wide dynamic range of radiation doses from a very low level to a very high level can be covered with a single detector and, at the same time, instantaneous high-intensity radiation and neutron doses can be measured.
One of the important constituents of the present invention is a stimulable phosphor used as a radiation detection medium. FIG. 35 is a schematic of the apparatus descried in D. J. Huntley et al., Nature, Vol. 313, 10, pp. 105-107 for reading the quantity of radiation accumulated in the stimulable phosphor. A stimulable phosphor sheet illuminated with a radiation in a different place is set on a reading table and then illuminated from the front with exciting light issued from an argon laser; the resulting emission of stimulated fluorescence is passed through bandpass optical filters centered at the frequency of stimulated fluorescence and subsequently detected with a photomultiplier tube; in accordance with its intensity, the detected fluorescence is digitized by a signal processor to determine the quantity of incident radiation.
In another conventional method, a small amount of stimulable phosphor is attached to the end of an optical fiber and illuminated with exciting light launched into the optical fiber and photons in the stimulated fluorescence that emits during the irradiation are counted to determine the incident radiation dose. This method was described by Kitaguchi et al. in JAERI-Conf 98-011, pp. 62-66 and the apparatus for implementing it is illustrated in FIG. 36. Due to the small amount of the stimulable phosphor used, it is difficult to increase the sensitivity of this method.
According to Radiation Protection Dosimetry, Vol. 65, No. 1/4, pp. 267-272, S. W. S. Mckeever et al. proposed a method in which a stimulable phosphor was illuminated with exciting light from a pulsed light source to read the dose of accumulated radiation. A schematic of this pulsed OSL (optically stimulated luminescence) system is shown in FIG. 37. After applying an impulse of exciting light on the timing shown in FIG. 38, the stimulated luminescence emitted in accordance with the life of fluorescence is amplified with a high-speed amplifier and photons in it are counted with a fast counter circuit to determine the accumulated dose of radiation. This method requires a high-speed measurement system including a photodetector.
A conventional method that can measure the dose of radiation and the position of measurement simultaneously with a simple apparatus was described by Maekawa et al. in Hoshasen, Vol. 21, No. 3, pp. 69-78. In this method, the detecting portion shown in FIGS. 39 and 40 is installed at each site of measurement and a plurality of such detecting portions are connected in series by an optical fiber for radiation measurement. Scintillator is used as a radiation detecting medium and the fluorescence emitted is detected with a wavelength shifting optical fiber and the radiation intensity as well as the position of measurement are determined from the difference between the arrival times of fluorescence at opposite ends of the wavelength Shifting optical fiber. A problem with this method is that if a radiation of high intensity is incident, not only its intensity but also the position of measurement is difficult to determine.